Carbon nanotubes have extremely favorable properties and therefore have many actual and potential applications, including electrical shielding, conducting polymer composites, and hydrogen storage reservoirs. Carbon nanotubes, however, suffer from the high cost of synthesis. Alternate carbon materials offer the potential of similar properties at significantly lower expense.
One such alternative material is graphite. Graphite is an abundant natural mineral and one of the stiffest materials found in nature (Young's Modulus of approximately 1060 gigaPascals (gPa)) with excellent electrical and thermal conductivity. It has better mechanical, thermal and electrical properties and lower density compared to clays. The lower cost of crystalline graphite ($1.5 U.S. dollars per pound ($/lb) to $1.6/lb and less than $5/lb for graphite nanoplatelets) compared to other conductive fillers, such as carbon nanotubes (about $100 per gram ($/g)), vapor grown carbon fibers (VGCF, $40/lb to $50/lb) and carbon fibers (about $5/lb to $6/lb), as well as graphite's superior mechanical properties compared to those of carbon black, makes graphite an attractive alternative for commercial applications that require both physical-mechanical property improvement and electrical conductivity of the final product.
Graphite is made up of layered sheets of hexagonal arrays or networks of sp2-carbon atoms. The sheets of hexagonally arranged carbon atoms are substantially flat and are oriented such that they are substantially parallel to each other.
Graphene is the aromatic sheet of sp2-bonded carbon that is the two-dimensional (2-D) counterpart of naturally occurring three-dimensional (3-D) graphite (Niyogi et al., “Solution Properties of Graphite and Graphene, Journal of the American Chemical Society, 2006, 128, 7720). The interlayer spacing of the graphene sheets in graphite is 3.34 Å, representing the van der Waals distance for sp2-bonded carbon (Niyogi et al.). Niyogi S., et al. also mention preparation of chemically-modified graphenes.
Novoselov K S, et al., “Electric Field Effect in Atomically Thin Carbon Films,” Science, 2004; 306: 666; Meyer J., et al., “The structure of suspended graphene sheets,” Nature, 2007; 446: 60-63; and Novoselov K S, et al., “Two-dimensional atomic crystals,” Proceedings of the National Academy of Sciences, 2005; 102: 10451-10453 mention methods of producing graphene.
Mack et al., “Graphite Nanoplatelet Reinforcement of Electrospun Polyacrylonitrile Nanofibers,” Advanced Materials, 2005; 17(1): 77-80, mention graphene has a Braunauer-Emmett-Teller (BET) theoretical surface area of about 2630 meter-squared per gram (m2/g).
Exfoliated graphite generally is an exfoliated or partially delaminated graphite having a BET surface area greater than BET surface area of graphite but less than the BET theoretical surface area of a single graphene sheet (i.e., less than 2630 m2/g).
Exfoliation or expansion of graphite is the process by which the distance between the graphene sheets in the graphite is increased, yielding a nanomaterial with an extremely large surface area. Such materials are useful for a variety of applications including, for example, in the formation of composites.
Conventional exfoliated graphene typically has a BET surface area of between about 25 m2/g to about 150 m2/g, depending on average particle size. Conventional exfoliated graphene is generally prepared by oxidation/intercalation of graphite to produce an expanded oxidized graphite, followed by an exfoliation step, such as rapid heating at high temperature. Recent work by Prud'homme et. al (WO 2007/047084) has shown that the well-known Staudenmaier synthesis using mixed concentrated sulfuric and nitric acids (Staudenmaier, L., Ber. Dtsh. Chem. Ges., 1898, 31, 1484), when combined with a high potassium chlorate concentration, can provide a very strongly oxidizing slurry for the oxidation/intercalation of graphite from which a highly exfoliated graphene can be formed. For example, WO 2007/047084 describes a graphite oxide preparation in which the weight ratio of potassium chlorate to graphite is between 20:1 and 8:1 (wt/wt). The ratio actually used in the examples of WO 2007/047084 was 11:1. In the original Staudenmaier process, the weight ratio of potassium chlorate to graphite is reported as 2:1 (see Chemical Abstract Number 0:95311). However the original Staudenmaier preparation does not produce the high surface areas needed for excellent properties.
A highly exfoliated graphene is mentioned in PCT International Patent Application Publication Number (PIPAPN) WO 2008/079585, and in U.S. Patent Application Publication Numbers (USPAPN) US 2008-0171824 and US 2008-0039573.
U.S. Pat. Nos. 3,676,315; and 4,004,988 mention, among other things, electrolytic production of aqueous sodium chlorate with electrolytic cells comprising graphite anodes. U.S. Pat. No. 4,773,975 mentions, among other things, a cathode compartment containing graphite particles in contact with aqueous sodium chlorate. U.S. Pat. No. 4,806,215 mentions, among other things, a process for production of chlorine dioxide and sodium hydroxide employing a cathode compartment comprised of graphite and aqueous sodium chlorate reagent.
One considerable disadvantage of known oxidation processes that ultimately produce exfoliated graphene is the required use of large quantities of chlorate, a material that is both expensive and has the potential for forming hazardous or explosive reactions. Such disadvantages render the known processes commercially unfavorable. Thus, while it is known how to expand graphite by making oxidized graphite, it is desirable to produce such expanded graphite (i.e., oxidized graphite) in commercial quantities in a more efficient, economic, and safe manner.